Over 1,000 people in Great Britain were questioned on their views on climate change as part of the Ipsos Mori poll. Photograph: Ann Pickford/Rex Features

Public conviction about the threat of climate change has declined sharply after months of questions over the science and growing disillusionment with government action, a leading British poll has found.

The proportion of adults who believe climate change is "definitely" a reality dropped by 30% over the last year, from 44% to 31%, in the latest survey by Ipsos Mori.

Overall around nine out of 10 people questioned still appear to accept some degree of global warming. But the steep drop in those without doubts will raise fears that it will be harder to persuade the public to support actions to curb the problem, particularly higher prices for energy and other goods.

The true level of doubt is also probably underestimated because the poll only questioned 16 to 64-year-olds. People over 65 are more likely to be sceptical, the researchers said.

"It's going to be a hard sell to make people make changes to their behaviours unless there's something else in it for them - [such as] energy efficiency measures saving money on fuel bills," said Edward Langley, Ipsos Mori's head of environment research. "It's a hard sell to tell people not to fly off for weekends away if you're not wholly convinced by the links. Even people who are [convinced] still do it."

John Sauven, the executive director of Greenpeace, said concern about fluctuations in public opinion have also prompted many environment groups to re-think their approach to campaigning - which has often focused on threats of climate disaster and making people feel guilty for their part in it.

"All of us have [talked about these changes]," said Sauven. "A lot of [recent] headlines have been grossly distorted, but that doesn't get away from the fact it's quite a complex issue, so we have got to talk about what is engaging and positive in terms of the response [which] can have many benefits to our society, for example energy security."

However, evidence that these events are behind the increased public uncertainty is mixed. Russ Lidstone, chief executive of the advertising agency Euro RSCG, which commissioned the Ipsos Mori survey, said their research among consumers found "great cynicism now as a result of questions in popular culture and regarding credibility of IPCC data". However a recent poll for the BBC suggested that these events have had less influence on British public opinion than the cold winter.

Adding to the confusion, Lidstone said that the agency's focus groups also showed the public was becoming "desensitised" to unseasonal weather and dramatic events like flooding.

Another likely factor was government action - or inaction - including the perceived failure of the climate talks in Copenhagen at the end of last year.Langley said: "That's something we do hear when we're doing dialogue work with the public: if this is a serious issue, then why aren't politicians taking it more seriously? But obviously that can be a way for them [voters] to detract responsibilty from themselves."

Just over 1,000 people in Great Britain were questioned on their views on climate change as part of Ipsos Mori's regular online omnibus poll on a range of issues. The results are weighted to reflect social groups and the split between men and women.

Thirty one percent of those polled said climate change was "definitely" happening, while 29% said it "it's looking like it could be a reality", and another 31% said the problem was exaggerated, a category which rose by 50% compared to a year ago. Only 6% said climate change was not happening at all, and 3% said they did not know.

Providing succor to those who believe that public opinion on the issue is largely fixed and that fears of a decline in public confidence are exaggerated, nearly half of the poll group said they "strongly agree" or "tend to agree" that climate change is a threat to all life on earth.

The response to the question on the causes of climate change could also reflect a more sophisticated understanding of climate science among the public than is sometimes assumed. Nearly one in five said climate change was "man-made", half that number said it was the result of "natural causes", and two-thirds said it was caused by a mixture of the two - putting the majority in line with scientists' conviction that the current global warming is predominantly man-made, but also influenced by natural factors such as the Sun's activity.

Climate change also dropped significantly down the list of voters' biggest concerns, ranking in the top three for only 17% of voters, nearly half the number in December 2007 soon after the IPCC's major assessment and another influential report on the the economics of climate change by Lord Stern for the UK government.

This lower priority could reflect the long-term nature of the impacts of climate change in the UK. In addition, people's priorities had changed significantly as a result of the economic downturn, focusing instead on unemployment and household budgets, said Lidstone.

However, Lidstone warned businesses not to react too quickly to changing public opinion and roll back moves towards cutting energy use and other sustainability policies.

"It is easy for some companies to believe sustainability will not be key to consumer decisions, but ultimately consumers will continue to want to know a brand or business track record on all elements of corporate responsibility," he said.

The BBC poll by Populus, published earlier this month, found that 25% of people thought climate change was not happening, compared with 15% in November last year.

"We know there's a physical effect of carbon dioxide in the atmosphere leading to higher temperatures, that's a question of physics; we know CO2 concentrations are at their highest for 6,000 years; we know there are observed increases in temperatures; and we know there are observed effects that point to the existence of human-made climate change," Miliband told the Observer. "That's what the vast majority of scientists tell us."

• This article was amended on 24 February 2010. The following sentence was deleted from the original because it was based on incorrect information provided: Another finding by the poll that hints at a growing lack of public confidence is a significant drop in those who said climate change was caused by human activities. One year ago this number was one in three, but this year just one in five people believes global warming to be man-made.